Normal functions of the immune system include defense against infections and detection and destruction of malignant or autoreactive cells. As the immune system ages and these capabilities decline, there is increased susceptibility to infections and cancer and an increased incidence of autoimmune disorders. The study of age-related changes in immune function is a relatively new area of investigation, which is limited by incomplete understanding of the complexities of immune mechanisms in general [1]. At present, most research is focused on defining normal changes in immune function at the basic science level, and much of the available data are from animal studies. In addition, there are no clinical interventions that have been definitively shown to counter immunologic aging.

Immunosenescence refers to the changes that occur in the immune system with increasing age. The clinical consequences of immunosenescence include an increased risk of infections, malignancy, and autoimmune disorders.

Infections — Pneumonia and influenza are among the top 10 causes of death in individuals aged 65 and older [2]. Nosocomial infections are also significantly increased in older adults. This could be the result of decreased immunologic function in addition to a decreased efficacy of vaccines in older adults. However, there are several other factors that are likely to contribute to increased infections in older adults, including malnutrition, comorbid conditions (eg, diabetes, chronic obstructive pulmonary disease [COPD]), diminished mucosal barriers, decreased cough reflex, and mechanical changes to the urinary tract system, among others [3].

The clinical presentation of infections in older patients may be different from that in younger patients. Older adults with severe infections tend to have fewer symptoms and fever is absent or blunted in 20 to 30 percent. This suggests a decreased ability to mount inflammatory cytokine responses in the face of infection. Signs of infection in older adults can be nonspecific and include falls, delirium, anorexia, or generalized weakness [4]. (See "Evaluation of infection in the older adult".)

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